1. Field of the Invention
This invention relates to a method of depositing metal particles on carbon nanotubes.
2. Discussion of the Prior Art
Carbon nanostructures have been used as metal catalyst supports in electrochemistry and fuel cells [see, for example H. Baughman et al, Science, 297, 787 (2002), G. Che et al, Langmuir, 15, 750 (1999), Z. Liu et al, Langmuir, 18, 4054 (2002), W. Li et al, Carbon, 40, 787 (2002), E. G. Steigerwalt et al, J. Phys. Chem. B, 106, 760 (2002) and T. Yoshitake, et al, Physica B, 323, 124 (2002)]. However, in the past, catalysts have been deposited on loose nanostructures, nanofibers and nanohorns just as on carbon black. When such carbon nanostructures were used in fuel cells, acceptable or better performance was always reported. However, it has been shown [see E. B. Easton et al, Electrochem. Solid-State Lett., 3, 359 (2000)] that when platinum on carbon black is used in fuel cells, only a fraction of the Pt-based catalyst is electrically connected to the backing electrode.
Carbon nanotubes are hydrophobic which makes it difficult for metal particles to adhere to them. E. Dujardin et al, Science, 265, 1850 (1994) have reported that carbon nanotubes could not be wet by liquids with a surface tension higher than 100–200 mn/m which means that most metals would not adhere to them. In order to improve adhesion of metals to nanotubes, two main approaches have been proposed, namely surface modification and sensitization activation. The former involves oxidation of the nanotube surface in order to create functional groups and increase metal nucleation [see Y. P. Sun et al, Acc. Chem. Res., 35, 1096 (2002) and T. W. Ebbesen et al, Adv. Mater, 8 155 (1996)]. Sensitization activation involves the generation of small nuclei, for example of Pd—Sn to promote metal deposits on carbon nanotubes [see Z. Liu et al, supra and L. M. Ang et al, Carbon, 38, 363 (2000)]. However, a need still exists for an efficient method of depositing metals, and in particular metallic catalysts on carbon nanotubes.